NO863017L - ALUMINUM ALLOY. - Google Patents

Description

This invention relates to anodes for electrochemical cells and more particularly to an aluminum alloy anode which has high energy and high electrochemical potential in the cells using strong alkaline solutions.

The basic requirement for a satisfactory anode for electrochemical cells, such as aluminium-air cells, is the end to

generate a high cell voltage while parasitic corrosion occurs. High voltage is essential as it enables the reduction of the number of cells necessary to construct a battery with a given voltage. Low parasitic corrosion or in other words high efficiency leads to higher energy density, i.e. lower anode costs and more importantly to

minimization of hydrogen gas formation. If this hydrogen gas is not vented out and diluted sufficiently, there is a possible danger of explosion. Furthermore, the venting operation itself causes possible problems with unsealing the batteries to prevent leaks.

Much previous research has relied on the addition of stannate to alkaline electrolytes to control the parasitic corrosion of the aluminum anodes. A preferred electrolyte that has been used consists of 4 molar sodium hydroxide, 1 molar dissolved aluminum and 0.06 molar sodium stannate (corrosion inhibitor). Nevertheless, there are a number of disadvantages with the use of such an electrolyte, namely the following: 1. Tin has a tendency for the electrolyte to deposit on the anode and thereby form a dendritic structure which eventually short-circuits the cell. 2. When the tin braids, its concentration in the solution decreases with the result that the inhibiting influence of the tin mesh is increasingly lost; 3. In the presence of stannate, the growth of aluminum hydroxide crystals is reduced and this results in the reduction of the efficiency of any regenerative crystallization unit.

Many studies have also been carried out on the composition of the anode. For example Pryor, et al in US Patent No. 3,189,486 describes an anode consisting essentially of aluminum and tin, preferably with indium. The beneficial effect of gallium, indium and phosphorus on an electrochemical activity is described in Despic, et al in US Patent No. 4,288,500. When added itl superpure aluminium, these elements raise anodic potentials.

Moden, et al in US Patent No. 4,107,406 describes an anode made of superpure aluminum containing small amounts of magnesium and gallium.

However, none of these above-mentioned alloys exhibit acceptable performance when used with a strong alkaline electrolyte without a corrosion inhibitor.

According to the present invention, a special

advantageous balance with potential and corrosion inhibition in an alkaline electrolyte when the anode is made of an aluminum-based alloy with special small additions of indium and at least either manganese or magnesium. In particular, the alloy according to the invention mainly consists of 0.01 to 0.20 wt% indium, at least either 0.01 to 0.25 wt% manganese or 0.01 to 1.5 wt% magnesium and the rest aluminum, i.e. an aluminum with at least 99.95% and preferably at least 99.99% purity. An anode made from the alloy of the present invention is particularly advantageous for use with a strong alkaline electrolyte, producing an excellent balance of potential and corrosion resistance without the need to add sodium stannate.

It has been found that the presence of manganese in the alloy is important in minimizing corrosion under charging conditions, while the presence of magnesium is beneficial in minimizing corrosion under non-charging conditions. Accordingly, the presence of both manganese and magnesium is preferred.

The alloy can also tolerate the presence of iron in amounts up to approx. 0.03% by weight without this leading to greatly increased corrosion. Other components that may be present in the alloy include silicon, tin, titanium and gallium.

A preferred alloy contains 0.05 to 1.0 wt% magnesium, 0.02 to 0.15 wt% indium, 0.02 to 0.2 wt% manganese and the balance is super pure aluminum. This alloy may also contain other minor components such as up to 0.005 wt% silicon, up to 0.005 wt% tin, up to 0.005 wt% titanium and 0.005 wt% gallium. These components and iron can either already be present in the super-pure basic aluminum or introduced as impurities with the alloying additions.

The alloy composition can be processed by a number of conventional casting methods, which include permanent laboratory molds, two-roll or two-belt casters and the usual direct cooling methods. An important feature of the invention is to ensure that a minimum level of the alloying constituents is in solution to at least a significant extent. According to this, the methods can include a heat treatment of the solution either on the ingot or the intermediate thickness step. This can be achieved by heat treatment for 8 hours at 600°C followed by a water cooling. The surface can either be processed hot or cold, but care must be taken during hot processing or intermediate heat treatment due to the possibility of precipitation of alloying elements during prolonged exposure to temperatures below the solution temperature. The presence of precipitated alloying elements in the microstructure can reduce the efficiency of the anode.

Certain preferred embodiments of the invention will now be illustrated by the following examples.

EXAMPLE 1

A series of alloys were prepared starting from superpure aluminum (at least 99.990% purity) and various alloying elements were added as indicated in Table 1. Alloys 1 to 8 were cast into 19 mm thick ingots and the ingots were scraped to remove approximately 0, 15 mm from each main surface. These articles were subjected to solution heat treatment for 8 hours at 600°C and following a water cooling, were cold rolled to form plates with a thickness of 3.2 mm. Alloy 9-14 was processed in the same way with the addition of a further cold rolling step to 14 mm followed by scraping but before the solution heat treatment. This additional operation had no measurable effect on the properties listed in Table 1.

The alloy plates thus formed were tested as anodes and their properties for generating cell voltages on open circuit and with external currents of 200 and 600 mA/cm<2> (EOC, E200, E600) at 60°C in 4 molar NaOH. Corrosion current was measured as weight loss at open circuit and with external current strengths of 200 and 600 mA/cm<2> (respectively ICOC, IC200, IC600) were also determined.

The alloy compositions and electrolytic results are listed in Table 1 below. From the above table it is clear that binary alloys were completely inadequate. From Table 1 it can also be seen on the basis of the high potentials and low corrosion current strengths that ternary and quaternary alloys containing Mn, Mg and In produce the most useful compromises of all the alloys tested. It is also clear that build-up of dissolved alloy in the electrolyte, e.g. as represented by testing alloy 8 in the electrolyte with 27 g/l of dissolved alloy does not affect the effect negatively.

EXAMPLE 2

In order to show the effect of manganese on the corrosion rate during charging of the alloy according to the invention, 8 different alloys were produced starting from superpure aluminum (at least 99.990% purity). These were cast containing two different concentrations of magnesium and indium and with and without manganese, using the same method as used for the alloys 1-8 of example 1. The corrosion at a charge of 200 mA/cm<2> was measured as in example 1, all measurements were taken three times and the mean value listed. The results are shown in table 2 below:

It will be seen from the above results that the quaternary alloys have the lowest corrosion rates.

EXAMPLE 3

To determine the effects of impurities on alloys according to the invention, a series of aluminum alloys were cast from superpure aluminum containing various concentrations of iron and silicon, together with relatively low concentrations of magnesium, manganese and indium. A high purity parent substance and a commercial pure parent substance were also used. The procedure used was the same as that for alloys 1-8 in Example 1.

The alloy compositions and results, measured as in Example 1 are shown in Table 3 below:

Claims (9)

1. Electrochemically active aluminum alloy, characterized in that it consists of 0.01 to 0.20% by weight of indium, at least one of 0.01 to 0.25% by weight of manganese and 0.01 to 1.5% by weight -% of magnesium and the rest aluminum with a purity of at least 99.95%. 2. An alloy according to claim 1, characterized in that it contains both 0.01 to 0.25 weight-% manganese and 0.01 to 1.5 weight-5 magnesium. 3. An alloy according to claim 2, characterized in that the aluminum has a purity of at least 99.99% <.> 4. Electrochemically active aluminum alloy, characterized in that it essentially consists of 0.01 to 0.20 wt% indium, 0.01 to 0.25 wt% manganese and the rest aluminum with a purity of at least 99.95% . 5. Electrochemically active aluminum alloy, characterized in that it essentially consists of 0.01 to 0.20% by weight of indium, 0.01 to 1.5% by weight of magnesium and the rest aluminum with a purity of at least 99, 95% <.> 6. An alloy according to claim 1, characterized in that it contains iron in an amount of up to 0.03% by weight. 7. Electrochemically active aluminum alloy, characterized in that it essentially consists of 0.02 to 015 wt% indium, 0.05 to 1.0 wt% magnesium, 0.02 to 0.2 wt% manganese and the remainder aluminum with a purity of at least 99.990%. 8. An alloy according to claim 1, characterized in that it has been solution heat treated. 9. An anode for a primary electrochemical energy source formed from the aluminum alloy according to claim 1.